12 - Macroscopic Coherent States of Excitons in Semiconductors

Summary

Abstract

Initially put forward by Moskalenko and Blatt et al., the idea of a possible Bose–Einstein condensation (BEC) of excitons in semiconductors has attracted the attention of both experimentalists and theoreticians for more than three decades. At different stages of this long history, the results of their efforts have been described and discussed in review articles. A brief introduction and summary of the main qualitative conclusions of this older work is presented here (Sections 1 and 2), followed by a more detailed discussion of some more recent developments (Sections 3 and 4).

Electronic Excitations in Semiconductors

Schematically presented in Fig. 1 is the typical electronic spectrum of a semiconductor: two bands (or two groups of bands) of continuous spectrum – conduction (c) and valence (v) – separated by the energy gap E_g = E_c,min – E_V,max. In the ground state, all of the states in the valence band(s) are occupied by valence electrons of the semiconductor, and all states in the conduction band are empty.

The lowest single-particle electronic excitations are an additional electron (e) in the conduction band or a single empty state – a hole (h) – in the valence band. Both of these excitation types are mobile fermions (spin = 1/2) characterized by effective masses, m_e and m_h, and effective charges e_e = e and e_h = −e, respectively. Here e is the usual (negative) elementary charge.